In recent years, a radar device (radar system) serving as a kind of wireless device is sometimes mounted to an automobile for the purpose of safe driving support or the like (for example, collision prevention). As an on-vehicle radar device of this kind, there is a device that uses a laser radar as a sensor or a device that uses a millimeter wave radar as a sensor. However, a sensing capability (resolving power) of the laser radar is liable to change due to external factors such as dirt on a vehicle and weather conditions. Therefore, in recent times, there has been more often employed a millimeter wave radar whose resolving power is hard to change due to the external factors and which can stably exhibit a desired resolving power at all times. Note that, the millimeter wave represents an electromagnetic wave having a wavelength of from 1 to 10 mm and a frequency of from 30 to 300 GHz.
In the above-mentioned millimeter wave radar, a waveguide tube slot antenna (referred to also as “waveguide tube slot array antenna”) is sometimes used as an antenna part for transmitting or receiving a radio wave. As a waveguide tube slot antenna of this kind, for example, in Patent Literature 1, there is disclosed a waveguide tube slot antenna including: a radiating slot body structure in which a radiating slot plate having an infinite number of radiating slots and a plurality of waveguide walls defining a plurality of waveguides are integrally molded with a resin and in which a metal plated coating film is formed at least on a defining surface of the waveguides; and a radiating back plate firmly attached to the radiating slot body structure. The radiating back plate is formed of metal, or is formed of a resin in which a metal plated coating film is formed at least on an inner surface thereof.
As the antenna part of the above-mentioned millimeter wave radar, the waveguide tube slot antenna disclosed in Patent Literature 2 may also be used. The waveguide tube slot antenna is formed by forming a plurality of slot-like antenna elements (radiating slots) in one surface of a metallic hollow waveguide tube (rectangular waveguide tube) that is seamless in its transverse section.
The waveguide tube slot antenna can be used not only as the antenna part for transmitting or receiving a radio wave having a millimeter wave band as described above but also as an antenna part for transmitting or receiving a radio wave having a centimeter wave band (radio wave having a wavelength of approximately from 10 to 100 mm and a frequency of from 3 to 30 GHz). Examples of the waveguide tube slot antenna that uses the radio wave having a centimeter wave band can include a wireless transmission system such as a wireless LAN or Bluetooth (trademark).